Rotary offset printing machines have been used for a number of years. The basic mechanisms, principles, and steps of operation for modern rotary printers include chemically forming an image on a thin metal image plate. The thin image plate therearound is attached around the circumference of a plate cylinder. Ink and a water solution are applied by rollers to the respective chemically treated areas that are to form an image on the image plate as it rotates with the plate cylinder. The plate cylinder rolls the image plate against a blanket cylinder offsetting a reverse image around the circumference of the blanket cylinder. A sheet of print paper or other material to be printed is fed into the press and gripped by an impression cylinder. The impression cylinder pulls the paper into rolling contact between the blanket cylinder and the impression cylinder. Under rolling pressure between the two cylinders, the image is imprinted from the blanket cylinder onto the paper. The imprinted image is the reverse of that on the blanket cylinder so that it is appears as originally formed on the image plate. After the paper is imprinted, it is removed from the impression cylinder gripper and transferred either to a collection tray if printing is finished, or to another impression cylinder, if additional colors or images are to be applied. The subsequent impression cylinder grips the print paper from a transfer gripper and rolls the paper against a subsequent blanket cylinder for additional printing.
In multi-color offset printers, each color is applied as a series of minute dots or patterns. It is extremely important to precisely locate or register the paper as it is gripped by each of the impression cylinders, so that each subsequent matrix of colored dots can be properly located and coordinated with respect to other color dots to form the desired image. Quality printing requires precise location of the dots within thousandths of an inch of each other. An error in alignment of a few thousandths of an inch can produce a blurred image or an image with improperly mixed and overlapping color dots.
In the past, multiple color offset rotary printers accomplished this precise registration through careful attention to the transfer of the paper using precisely manufactured fixed diameter transfer cylinders. Usually, the transfer cylinders are large enough to carry two sheets of print paper spaced end-to-end around the circumference of the cylinder. The rotation of the transfer cylinders had to be carefully timed with respect to the rotation of the impression cylinders so that: the paper when picked up by the transfer cylinders from one of the impression cylinders was carried around the transfer cylinder at precisely the correct speed and distance so that it was gripped by a subsequent impression cylinder precisely in the correct location for registration. The grippers for each impression cylinder had to be adjusted until the dots were printed precisely at the desired location.
The use of transfer cylinders has been important because of the extreme criticality of precise registration. The cylinders, once formed, have a fixed diameter and can be rotated through gears at a fixed speed. Repeatable transfers are thus made possible. However, this structure is complex and expensive. Further, it introduces associate problems. For example, smearing can result because the printed surface of the paper being transferred is directed inward on each transfer cylinder. Thus, the printed surface of the sheet faces outward toward the blanket cylinder when it is gripped by the next impression cylinder. Special coatings, special non-stick screens, and even complex systems for air cushioning the paper as it is carried around the transfer cylinder have been employed in order to minimize this smearing problem.
The cost of manufacturing multiple color offset printers has been very high because of the complexity of multiple transfer gripping mechanisms, large precision-built transfer cylinders, and non-smear mechanisms. Further, because of the need to properly adjust registration of the paper as it is received by each impression cylinder, transferred to each transfer cylinder and then received by each subsequent impression cylinder, the time and expense to set up any given multiple color offset printing job has been substantial. It is not uncommon for an operator to spend a considerable amount of time setting up a job and to use over five hundred (500) trial printing sheets before proper registration is obtained for all of the color impression cylinders. As a result, multiple color offset rotary printing has not been economically feasible for most small printing jobs requiring less than several thousand copies.